The research plan for the combined clinical-research training program is organized into two phases. Phase I, which is described in detail in this proposal, will be initiated at the start of the award and will be completed by year 03 of the award. Using phase I research as a foundation, phase II of the training program will address one or more of the specific questions about condylar growth and muscle adaptation that arise from the first two years of the research project. The specific research problems that will be addressed in phase I concern (i) the role of the lateral pterygoid and superficial masseter muscles in the growth and adaptation of the mandibular condyle and (ii) the adaptations that occur within the lateral pterygoid and superficial masseter muscles in response to functional protrusion of the mandible. Laboratory rats will be used as experimental animals. The specific hypotheses that will be addressed are: (i) functional mandibular protrusion results in an increase in the tonic activity of the lateral pterygoid muscle and, to a lesser extent, the superficial masseter muscle and (ii) this muscle activity is associated with an increase in condylar growth. The specific purposes of the experiment are: (i) to examine adaptations within the lateral pterygoid and superficial masseter muscles to altered mandibular position using quantitative morphologic and physiologic techniques (i.e., histologic analysis of fiber length, histochemical analysis of fiber types and area, stimulated bite force, and contractile properties analysis) and (ii) to examine these findings within the context of morphologic variations that occur within the mandibular condyle related to cartilage cell proliferation and endochondral ossification (i.e., autoradiographic analysis of condylar growth and cephalometric analysis of mandibular growth and form).